Process for the production of a thermal shock tube, and the product thereof

ABSTRACT

A process for the production of a thermal shock tube is used to form a product that is utilized as a transmission device for connecting and initiating explosive columns, or as a flame conductor. The device is usually complemented by a delay element or used as a delay unit. The thermal shock tube uses a pyrotechnic mixture with low sensitivity to ignition by shock or friction, with low toxicity, and which generates a spark with superior thermal performance. The process utilizes continuous and separated dosing of the individual non-active components, in conjunction with the formation of the plastic tube, making the process safer, and with a more accurate dosing. The product maintains the advantages of current art pyrotechnic shock tubes relative to the shock wave propagating tube, e.g. larger transmission sensibility and sensitivity, propagation even with cuts or holes in the tubes, and low risk transport classification. The product has the additional advantages of using low toxicity components, use of ordinary, low cost, low adhesiveness polymers, the generation of a spark that propagates through knots, closed kinks or tube obstructions, and resistance to failure due to attack of components by hot explosive emulsions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to explosive signal transmissiondevices, and more particularly is a thermal shock tube and the method ofmanufacturing the shock tube.

2. Description of the Prior Art

Since at least the 1970's, low energy signal fuses known commercially as“non-electric detonators” or “shock tubes”, have been widely used forconnecting and initiating explosive charges in the mining and quarryingindustries. Such devices, marketed with brands like NONEL, EXEL, BRINEL,etc., came to be substituted for electric blasting caps ignited bymetallic wiring, and represented a revolution in the market ofdetonation accessories, due to the ease of connection and application,and to the intrinsic safety against accidental ignition by induction ofspurious electric current.

Current processes and products that use high explosives as components(hereinafter referred to as “conventional shock tubes”) are exemplifiedby the following:

1) U.S. Pat. No. 3,590,739 is the original reference for a conventionalshock tube. The reference describes a process of plastic extrusionforming a circular tube with an outer diameter varying from 2.0 to 6.0mm and an inner diameter varying from 1.0 to 5.0 mm. A secondaryexplosive powder, such as HMX, RDX or PETN, is introduced into its innerperiphery during formation of the tube. The resulting product is knownas a non-electric shock tube, and is marketed with trade names such asNONEL and EXEL. When initiated by a primary explosive blasting cap, aconventional shock tube generates a gaseous shock wave with a signaltransmission speed ranging from 1,800 to 2,200 m/s. Further improvementsinclude the addition of aluminum to increase specific energy andutilization of ionomeric polymers, like SURLYN, to increase adhesivenessof the powder.

2) U.S. Pat. No. 4,328,753 describes a conventional shock tube with twolayers: an inner layer made of a polymer which provides adhesiveness tothe explosive powder mixture, and an outer layer made of a polymer whichprovides mechanical strength. SURLYN is most suitable for the innerpolymer layer, and polypropylene, polyamide, or polybutene is used forthe outer layer. This product was an improvement over the original NONELtube, as SURLYN alone is expensive and has a low resistance to externaldamage.

3) European patent EP 027 219, and the related U.S. Pat. Nos. 5,317,974and 5,509,355 describe a single-layer shock tube, and its method ofmanufacture, in which the polymer is Linear Low Density Polyethylene(LLDPE) with minor quantities of an adhesive promoter. The tube is madeby extrusion of an initial tube with outer and inner diameters greaterthan that of the final tube. Then the tube is stretched in order toorient the LLDPE molecules, making a final tube with greater tensilestrength. All claims are for a minor amount of an adhesion promoter inthe polymer formulation, as it is well recognized in the art thatpowders have a low adherence to LLDPE. However, the best conventionalshock tubes continue to be made in two layers, and the inner layercontinues to be SURLYN, as even a low dislodgement of poorly adheredexplosive powder may lead to failures in signal propagation due todiscontinuities in the powder layer or by concentration of loose powderin the lower parts of the tube during field application.

4) U.S. Pat. No. 5,166,470, describes a single-layer tube of LLDPEsimilar to that of EP 027 219, but with an additional thin layer of ahydrophilic polymer, like Polyvinyl Alcohol (PVA), is deposited bypassing the plastic tube through a solution of polymer in a liquid, e.g.water, and drying the solvent. The aim is to make the tube lesspermeable to the hydrocarbons present in an emulsion explosive. Hotdiesel fuel is particularly aggressive to LLDPE, and prolonged contactof the tube with hot, diesel fuel-based emulsions causes failure insignal propagation. The PVA protective skin is fragile and does notadhere well to the LLDPE, and so a pretreatment with a cleaner (likechromic acid), with hot air or with an adhesion promoter (like VinamulEVA copolymer) is necessary.

A further development in low energy transmission fuses was the inventionof tubes that make use of pyrotechnic mixtures inside the tube, assubstitutes for high-explosive-containing powders. Currently, some ofthe processes and products with pyrotechnic mixtures, hereinafterreferred to as “pyrotechnic shock tubes”, are the following:

1) Brazilian patent PI 8104552, from the applicant of the presentpatent, is the original reference for the pyrotechnic shock tube. Itdescribes a process of plastic extrusion forming a circular tube with anouter diameter ranging from 2.0 to 6.0 mm, and an inner diameter rangingfrom 1.0 to 5.0 mm. A powder of pyrotechnic mixture of K₂Cr₂O₇+Al or Mg,Fe₂O₃+Al or Mg, or Sb₂O₃+Al or Mg, Sb₂O₅+Al or Mg or O₂+Al or Mg, isintroduced in the inner periphery of the tube during formation of thetube. The resulting product is designated as a pyrotechnic shock wavetube, and is marketed with the trade name BRINEL. When initiated by aprimary explosive detonator, such a tube generates an aluminothermyreaction without gas releases, and develops a plasma for energytransmission.

2) U.S. Pat. No. 4,757,764 describes a non-electric system forcontrolling an initiation signal in blasting operations using a plastictube with pyrotechnic delay mixtures adhered to its interior. Thisdevice uses low speed reactions, with much slower speeds than those ofconventional shock tubes and detonating cords, with the object being touse predetermined lengths of tube to obtain a delay time in themilliseconds range, the tube being substituted for a conventional delayelement. The blasting caps connected to the plastic tube are necessarilyinstantaneous, without delay elements in the cap. There was therefore noattempt by the inventor to optimize the thermal action of a spark, norto eliminate toxic components, nor to guarantee the crossing throughrestrictions in the tube. Similarly, there was no effort to reduce thesensitivity of the mixture to friction and mechanical shock, or toaddress the adhesiveness of the mixture to the tube. There was also noconsideration of the resistance to attack by hot hydrocarbons from theemulsion explosive. It is evident, by the patent's descriptive report,and from all of the examples, that its use as a delay element is limitedto the range of tens of milliseconds, which is not adequate for most ofthe delays required in field practice.

Signal transmission tubes are usually complemented with the insertion ofa delay blasting cap in the tip of the tube. The blasting cap is made ofa metal cap containing two layers of explosive powder pressed inside.The bottom layer is a secondary high explosive, and the upper layer is aprimary, flame-sensitive explosive. The cap further includes a delayelement consisting of a metallic cylinder containing in its interior acompacted column of powdery pyrotechnic delay mixture and, frequently,an additional column of pyrotechnic mixture sensitive to the heatgenerated by the tube's shock wave.

The process for the manufacture of a conventional shock tube, as well asthe resulting product, presents the following disadvantages:

a) The production of the tube loaded with explosives (RDX, HMX or PETNare toxic and dangerous) offers risks both of accidental explosions andin the handling toxic products, requiring special care and protection inthe production line. The fact that molecular explosives are used impedesthe dosing of non-active components during the extrusion of the tube.

b) In the conventional shock tube, the reaction products are basicallyhot gases which, when leaving the final extremity of the tube, expandwith loss of heat, such heat loss inhibiting the ignition of thepyrotechnic delay mixture. Slower delay powders are particularlyinsensitive to the shock tube output. It is therefore necessary eitherto add an additional column of a sensitive pyrotechnic mixture to givecontinuity to the explosive train or to use pyrotechnic mixtures moresensitive to heat and with larger column length. As a consequence, thefinal product has greater production costs, and the processing andhandling of the pyrotechnic mixture entails significant accidentalignition risks.

c) The adherence of crystalline explosives (RDX, HMX or PETN) in plastictubes is low, demanding special manufacturing processes and the use ofspecial, and expensive, polymers, usually ionomeric polymers such asSURLYN, in order to minimize the concentration of loose powder inportions of the tube and to insure uniformity of distribution. Lack ofadherence of LLDPE is particularly noteworthy. It is significant thatthe best known commercial brands of shock tube continue to use a twolayer tube, with SURLYN as the inner layer, in spite of the efforts toimprove polymer adhesiveness by changes in polymer formulation.

d) Conventional shock tube loading lacks sufficient critical mass andcritical diameter to properly propagate a shock wave by classicaldetonation theory. The finding of the late Dr. Persson, inventor of theoriginal shock tube, was that the shock wave is continuously sustainedby dust explosion of the explosive powder dislodged by deformation ofthe plastic duct caused by the shock wave behind the reactive front. Dueto this feature, a conventional shock tube fails if there is a cut or aclose restriction in the inner duct, dispersing the shock wave. In fieldpractice, if unexpected cuts, stretching, knots, holes, or closed kinksunexpectedly appear in the tube, the tube can fail to propagate.

e) Conventional shock tubes are sensitive to the effect designated inthe industry as “snap, slap, and shoot”. An unexpected ignition canoccur if the tube is stretched causing rupture, in particular conditionsof mechanical energy release, as recognized in an article presented inthe 28th Annual conference of the ISEE, Las Vegas, 2002, and in allcatalogs and technical bulletins of conventional shock tubes.

f) Conventional shock tubes are classified for transport purposes as anexplosive in many countries, which results in additional costs anddifficulties for transportation, especially after the increase indangerous products regulations resulting from the fight againstterrorism.

g) Conventional shock tubes can fail to propagate after prolongedunderwater exposure above 2 bar pressure, as is often found in fieldpractice, due to the hydrophilic characteristics of the ionomeric resinslike SURLYN.

h) Tubes manufactured with SURLYN alone have a low tensile strength, anda low resistance to abrasion, kinks, knots, etc., demanding co-extrusionof an additional outer layer of polyethylene. This improved processstill includes the use of expensive SURLYN.

i) Conventional explosive powders lack sufficient activation energy topropagate in case of contamination of the tube interior by hothydrocarbons (most likely diesel fuel) from explosive emulsions.Polymers, including LLDPE, are quite susceptible to aggression. Minorquantities of adherence-improving additives, typically EVA copolymers,are even more subject to attack by volatile fractions of diesel fuel. Anadditional skin of hydrophilic polymer like PVA is needed, but abrasionresistance of the skin, particularly in the rough environmentalconditions found in field practice, is remarkably bad, causing removalof the skin and failures of the tube.

j) According to the specifications published by the manufacturers,conventional shock tube speeds of deflagration range from 1,800 to 2,200m/s, or within 10% of a mean speed of 2,000 m/s. This relatively broadrange interferes with the accuracy of the delay element timing. U.S.Pat. Nos. 5,173,569, 5,435,248, 5,942,718, and Brazilian patentPI9502995, from the author, all use a shock tube as the initiator of anelectronic delay blasting cap. Such caps are characterized by a highlyaccurate electronic delay element. However, the timing error of acertain length of tube is added to the intrinsic timing error of theelectronic circuit. In a typical tube length of 21 m, as used in openpit mining, the error would be within +/−1 ms, while the intrinsic errorof the electronic circuits is typically within +/−0.1 ms.

k) Conventional shock tube deflagration generates substantially gaseousreaction products, sustaining a shock wave that quickly disperses mostof the released thermal energy, through the expansion of the gases asthey leave the tip of the tube. For this reason, a conventional shocktube output is unable to ignite low flame-sensitive delay mixtures,demanding an additional, highly flame-sensitive, igniter element forignition of the slower delay elements. Highly flame-sensitive mixturesare usually also highly sensitive to mechanical shock, friction andelectrostatic discharge, increasing the risks of accidental detonation.The additional element also increases the manufacturing costs.

A pyrotechnic shock tube, as disclosed in Brazilian patent PI 8104552,from the applicant of the present patent, has the followingdisadvantages:

A) Pyrotechnic mixtures use toxic components (K₂Cr₂O₇, Sb₂O₃, Sb₂O₅) andflammable solvents, demanding recycling of the solvents, and creatinghandling issues and requiring appropriate waste disposal.

B) The process of extrusion of the plastic tube includes the dosing of apreviously prepared sensitive pyrotechnic mixture during the formationof the plastic tube, with safety risks in handling and processing.

C) Like a conventional shock tube, a pyrotechnic shock tube does notresist aggression from the hydrocarbons present in emulsion explosives,and prolonged exposure leads to failures in propagation.

D) Mixtures of O₂+Al or Mg were not shown to be feasible in practice,due to the loss of gases in the production and use of the product.

E) Mixtures of Fe₂O₃+Al or Mg were also not shown to be feasible inpractice, due to the low sensibility of these pyrotechnic mixture to theignition stimulus of blasting caps and a high rate of propagationfailures. The fundamental cause proved to be the components high Tammanntemperature.

F) Giving the limitations presented by shortcomings D and E, the onlyremaining options were highly toxic, highly friction and shock sensitivemixtures of K₂Cr₂O₇, Sb₂O₃, and Sb₂O₅ with Al or Mg.

G) The reaction products formed in the aluminothermy reactions, Al₂O₃,K₂O, Sb, antimony oxides, Cr₂O₃, necessarily solids by the claimedlimitations, have low thermal conductivity, which inhibits the ignitionof slower, low sensitive delay elements.

H) Like conventional shock tubes, the powdered pyrotechnic mixture alsopresents a low adherence to the tube polymer, particularly in LLDPE.

I) Pyrotechnic mixtures are not optimized to allow propagation throughclosed knots, cuts or kinks.

The system for control of an initiation signal in blasting operationsdisclosed in U.S. Pat. No. 4,757,764 presents the followingdisadvantages:

Aa) As with the original pyrotechnic shock tube, the process alsoincludes the dosing of a previously prepared sensitive pyrotechnicmixture, during the formation of the plastic tube, with safety risks inhandling and processing.

Bb) The system makes use of direct tube-to-tube connections forsupplying a time delay exclusively through a predetermined length oftube, and is limited to fast delays, in the range of tens ofmilliseconds, while field blasting operations demand delay timing up to10 s.

Cc) The powdered mixtures, containing no adherence additive, present alow adhesiveness to the tube polymer, requiring the use of expensivematerial, like SURLYN or silicone, as can be seen in all of the examplesin the descriptive report.

Dd) As the inventor's aim was a system of delay obtained through a tubewith substantially reduced speed, eliminating the delay element, anddirectly igniting the highly sensitive primary explosive inside theblasting cap, there was no optimization of the thermal performance of atransmission signal. A low speed mixture lacks the energy to directlyignite slower, low sensitive delay mixtures, and to propagate throughclose kinks, knots or cuts.

SUMMARY OF THE INVENTION

The present invention is a thermal shock tube and the method ofmanufacturing the shock tube. The shock tube is used as a signaltransmission device for connecting and initiating explosive columns, oras a flame conductor. The device is usually complemented by a delayelement, or it can be used as a delay unit. The shock tube uses apyrotechnic mixture with low sensitivity to ignition by shock orfriction, with low toxicity, which generates a spark with superiorthermal performance. The manufacturing process utilizes continuous andseparated dosing of the individual non-active components, in conjunctionwith the formation of the plastic tube, making the process safer andyielding a more accurate dosing. The resultant product maintains theadvantages of current art pyrotechnic shock tubes relative to the shockwave propagating tube, i.e. larger transmission sensibility andsensitivity, propagation even with cuts or holes in the tubes, and lowrisk transport classification. The shock tube of the present inventiongives the following additional advantages: use of low toxicitycomponents, use of ordinary, low cost, low adhesiveness polymers,generation of a spark that propagates through knots, closed kinks ortube obstructions, and resistance to failure by attack of components ofhot explosive emulsions.

The focus of the present invention is to obtain desirablecharacteristics in the polymers that form the tube, but not to optimizethe pyrotechnic mixtures formulation, in order to use ordinary, low costpolymers. The new approach is also multipurpose, i.e., to obtain thegreatest possible number of desirable characteristics through theformulation of the pyrotechnic mixture. The process and product fromthis invention have the following advantages over the current art shocktubes:

-   -   The thermal shock tube employs an optimized pyrotechnic mixture        with low toxicity.    -   The process allows the continuous dosing and mixture of two        non-active components during the extrusion process, the        components being essentially insensitive to friction and shock        before mixture, thereby substantially reducing the probability        of accidental initiation in handling, and, in case of ignition        of the tube during production, minimizing the damages by the        deflagration of a very small amount of mixture.    -   The process yields a safer pyrotechnic mixture, with smaller        sensibility to friction and mechanical shock, by covering the        oxidizer components with a desensitizing additive.    -   The pyrotechnic mixture has an excellent adherence to the        plastic tube, using low cost, common polymers, including LLDPE,        and avoiding tube portions with lack or excess of charge.    -   The product maintains some advantages of the current pyrotechnic        shock tube in relation to a conventional shock tube, e.g. a        larger sensibility and sensitivity of propagation, propagation        even with cuts or holes, and low risk classification for        transport.    -   The spark of signal transmission is formed as much by gases as        by melted metals, and so it crosses knots, closed kinks or        obstructions in the tube, and presents an optimized heat        transport by thermal conduction and convection, igniting less        sensitive, slower delay columns directly.    -   The thermal shock tube resists environmental exposure to marine        diesel fuel present in hot explosive emulsions, maintaining        functionality even after 72 hours of exposure at high        temperature (65° C. for 24 h+40° C. for 48 h in pure marine        diesel).    -   The thermal shock tube has propagation speed accuracy within        +/−1.67% from the mean speed, i.e., an error of +/−20 m/s in        1,200 m/s, adding to electronic delay detonators only +/−0.3 ms        of error in a 21 m long tube.

Objectives considered during development of the present inventionincluded:

-   -   Elimination of poisonous components of the pyrotechnic mixture;    -   Improvement in the adherence of the mixture to the inner surface        of the tube;    -   Desensitization of the mixture to shock and friction;    -   Decrease in handling risks of the pyrotechnic mixture;    -   Substitution of automated manufacturing processes for the        pyrotechnic mixtures that were formerly labor-intensive,        including grinding and re-crystallization with dangerous        solvents, and handling the pyrotechnic mixture by automated,        risk-free, and environmentally-safe processes;    -   Generation of an optimized spark with excellent heat transfer by        conduction and convection without dispersion of heat by gas        expansion;    -   Production of a tube with functionality after exposure to hot,        diesel fuel-based explosive emulsions up to 65° C. for 3 days.

These and other objects and advantages of the present invention willbecome apparent to those skilled in the art in view of the descriptionof the best presently known mode of carrying out the invention asdescribed herein and as illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the manufacturing process for thethermal shock tube of the present invention.

FIG. 2 shows the thermal shock tube spark as it leaves the tube tip.

FIG. 3 shows the basically gaseous products of a conventional shock tubewhen leaving the tube tip.

DETAILED DESCRIPTION OF THE INVENTION

One of the fundamental concepts for the understanding of the presentinvention was described by the Russian chemist Tammann. According to histheory, the vibrational energy needed to start an oxidation-reductionreaction among solid substances is largely available at the temperatureequivalent to half the melting point of the substance, in the absolutescale (K). This temperature of Tammann explains why certain componentsmake pyrotechnic mixtures quite sensitive to heat to flame andmechanical shock, while other ones are quite difficult to start anpropagate. For example, mixtures of powdered aluminum, whose temperatureof Tammann is 193° C. and ferrous-ferric oxide, Fe₃O₄, whose temperatureof Tammann is 632° C. are particularly difficult to start and propagate,while mixtures of powdered aluminum and potassium chlorate, whosetemperature of Tammann is only 47.5° C., is especially dangerous. One ofthe invention objectives is to obtain enough activation energy to ensurethe initiation and propagation of the pyrotechnic reaction even withcontamination of the interior of the tube by hydrocarbon fuel comingfrom the explosive emulsion, such contamination decreasing the enthalpypyrotechnic reaction. Examples of low-Tammann temperature substancessuitable for the pyrotechnic mixture are potassium perchlorate,potassium chlorate, antimony trisulfide, sulfur, potassium nitrate,ammonium perchlorate, sodium chlorate, or any other substance whosetemperature of Tammann is adapted to this purpose.

A pyrotechnic reaction that generates products with high thermalconductivity and thermal convection coefficient will allow betterpropagation continuity, and will ignite delay elements with greaterthermal efficiency, allowing the use of smaller, slower delay columnswithout additional ignition elements. As relevant oxidation-reductionreactions, we have:

-   8 Al+3 Fe₃O₄    4 Al₂O₃ (solid)+9 Fe (liquid) or,-   2 Al⁺Fe₂O₃    Al₂O₃ (solid)+2 Fe (liquid)    where the melted metallic iron supplies an excellent heat transfer,    as much by thermal conduction as by convection.

The generation of solid or liquid products will not allow propagationthrough knots, kinks, restrictions, etc. It is necessary that enough gasvolume be generated to allow the elastic expansion of the polymer aroundthe fold or restriction, forcing the propagation of the spark. However,the gas volume cannot be excessive, or there will be dispersal of thesolid and liquid products of the spark in the tip of the tube, combinedwith the gaseous expansion, that will provoke the loss of the thermalenergy necessary for ignition of the delay element. Examples ofcomponents found to be appropriate for gas generation are antimonytrisulfide, potassium perchlorate, potassium nitrate, sodium nitrate,ammonium perchlorate, sodium perchlorate, etc.

Certain products have lubricating properties and superficial adherenceproperties, which reduce the effects of friction and mechanical shock ofthe mixture, and provide adhesiveness even to difficult polymers likepure LLDPE. Examples of such products are: talc (magnesium and aluminumhydrosilicate) and graphite.

Another unique feature of the process of the present invention is thatthe mixture of the oxidizers and additive is done separately from thefuels or reduction agents. The final active mixture is obtained in theplastic extruder, in an automated, continuous or semi-batch process, sothat just a very small amount of pyrotechnic mixture is formed at anyinstant. This minimizes the hazard of an accidental ignition of the tubeduring production.

In order to allow propagation through cuts or holes accidentally made inthe tube during use, the spark is constituted as much by products ofhigh heat transfer as by gaseous products so that the heat transferallows continuity of the pyrotechnic signal transmission so as toprovide the mechanical impulse for releasing the spark from the openportion of the tube.

The development of the optimized formulation for the thermal shock tubewas accomplished by several practical tests. For these tests,formulations of powdered pyrotechnic mixtures were dosed by spraying inthe inner diameter of the tube with melted pure LLDPE in an extruder.The tube was cooled, and stretched to obtain a 3.1 mm outer diameter,1.4 mm inner diameter flexible tube. Conventional SURLYN shock tubes aswell as prior art pyrotechnic shock tubes were sampled and tested as acomparison.

The tests used are as follows:

1) Speed of propagation test: A tube portion with a length of 5 m isplaced between two optical sensors linked to a precision chronometer.When the tube is ignited, the spark passes the first sensor to triggerthe chronometer. When the spark passes the second sensor, the timing isended. The propagation speed is obtained by dividing 5 by the timemeasured in seconds.

2) Kink propagation test: In 10 samples, the tube spark should propagatethrough 10 closed 180° folds spaced by the same distance. This smallestdistance among the following—m, 50 cm, 30 cm, 20 cm, and 10 cm—in whichall 10 samples propagate completely, without failure, is recorded as“minimum distance between kinks”.

3) Tight knot propagation test: a 1 m long tube sample is single-knottedin its middle section, and the tube extremities are held by ahydraulically-driven traction device, with a loading cell attached tomeasure the tensile strength to which the knotted tube is submitted. Thetube is ignited, and the maximum load in which five successive samplespropagate through the knot is recorded. The higher the maximum load, thebetter the ability of the tube to propagate through tight knots whichcould accidentally be made in field use. This test was performed forsingle-layer shock tubes, as well as for double-layer (LLDPE and SURLYN)conventional shock tubes, for comparison.

4) Low energy detonating cord initiation: 100 samples of 1 m long tubesare connected to a line of detonating cord with a core loading of 2grams/m of PETN, through a “J” type connector, and the detonating cordis initiated. The number of tubes which fail to propagate is recorded as“percentage of failures in initiation by 2 grams/m detonating cord”.

5) Mechanical Shock Sensibility: A sample of the pyrotechnic mixturepowder is submitted to a known weight falling hammer, free-falling froma known height. The energy that causes 5 successive samples todeflagrate is recorded. The energy is calculated by the formula E=m×g×hwhere m is the mass of the weight in free fall, g is the localacceleration of gravity, and h is the minimum height for ignition.

6) Slower delay sensibility: A delay element of 8.3 seconds delay time,with a 24 mm long column of pressed delay powder, containing slow delaymixture, without any additional igniting mixture, is placed at the endof a PVC tube with a 6 mm outer diameter, with variable length, with thetip of a 1.0 m long thermal shock tube, aligned in the other extremity.When the thermal shock tube is ignited, the spark should cross the freespace from the hose interior and start the delay element. The larger thelength of the hose in which the elements always ignited, the better thethermal performance of the spark. The largest hose length for ignitionin 5 successive samples is recording as “sensibility of the slow delayelement”.

7) Tube-to-tube “air gap”: A 3 m long thermal shock tube istransversally cut and the tube halves are moved a measured distanceapart, maintaining their alignment through an aluminum guide in“half-pipe” format. The largest distance that the spark can cross thegap between the tube portions and initiate the second portion in 5successive samples, is recording as “all-fire air gap”.

8) Initiation after exposure to the hot explosive emulsion: 30 samplesof 12 m long thermal shock tube, with the ends sealed by a rubber plugand a crimped aluminum cap, as is usual in the industry, are dipped in65° C. hot bulk explosive emulsion with marine diesel oil as fuel, andplaced in a lab stove at 65° C. for 24 hours. After this period, thestove has its thermostat lowered to 40° C., and the samples stay in theemulsion for 48 more hours, totaling 72 hours of exposure. The tubes areignited and the percentage of failed tubes is recorded as “failuresafter exposure to the hot emulsion”.

9) Adherence of the mixture to the tube: 10 tube samples 5 m long areweighed in an analytical scale with an accuracy of 0.0001 g. Theinteriors of the tubes are flushed by compressed air with a flow rate of0.3 Nm³/minute for 2 minutes, to remove the non-adhered powder. Thetubes are weighed again and the weight is recorded. The interior of thetubes is washed with a flow of sodium hydroxide aqueous solution fordissolution of the aluminum and perchlorate, and iron oxide and talc,eliminating the adhered powder. The empty plastic tube is weighed. Afterdetermination of the tube's inner diameter the surface area iscalculated and the free powder load by area rate, the adhered powderload by area rate, and the percentile rate of free powder mass by totalpowder mass are calculated.

The test results are consolidated and summarized in the followingTable 1. TABLE 1 Practical Tests Results Low Initiation after Minimumgap Tight energy exposure to Adherence of between kinks in knotdetonating Slower the hot the mixture to Speed of kink propagationpropa- cord Shock Delay all-fire explosive the tube (% of Formulationpropagation test gation initiation Sensibility Sensibility air gapemulsion free powder) Al 65%  750 m/s Failure at any  3 f-kg 8% 9.2 N  8cm  80 mm 25%   5% Fe₃O₄ 17% distance KClO₄ 17% Talc 1.0% Al 50% 1170m/s  1 m  8 f-kg Zero 9.2 N 16 cm 100 mm zero  3.8% Fe₃O₄ 24.5% KClO₄24.5% Talc 1.0% Al 40% 1260 m/s 30 cm 11 f-kg Zero 9.2 N 22 cm 120 mmzero  4.0% Fe₃O₄ 27.5% KClO₄ 31.5% Talc 1.0% Al 30% 1290 m/s 30 cm 12f-kg Zero 9.2 N  5 cm  15 mm 15%  3.2% Fe₃O₄ 32.5% KClO₄ 36.5% Talc 1.0%Al/K₂Cr₂O₇ 1000 m/s Failure at any  3 f-kg Zero 3.8 N  8 cm 100 mm 30%18.3% Standard distance for the prior-art pyrotechnic shock tube Mixture2000 m/s  1 m  2 f-kg Zero 3.8 N Fails to  10 mm not not HMX/AI ignite,even performed performed Standard at zero for the distance convention alshock tube single layer Mixture 2000 m/s 50 cm  8 f-kg zero 3.8 N Failsto  10 mm not not HMX/AI ignite, even performed performed Standard atzero for the distance convention al shock tube double layer

According to the test results in Table 1, the formulationAl/Fe₃O₄/KClO₄/Talc in the respective percentiles 40/27.5/31.5/1.0 isoptimal for the shock tube of the present invention. A high content ofaluminum fuel with 65% Al, with a corresponding lower speed of 750 m/s,means an insufficient spark performance in the propagation through kinksand knots, and a very low sensibility of the slow delay element. On theother hand, a very low aluminum fuel content, as in the formulation30/32.5/36.5/1.0, will generate a very high gaseous volume, dispersingthe spark products at the tube tip, reducing the sensibility of the slowdelay element and the “all-fire air gap”. The results confirm theefficacy of the talc in improving the adherence of the mixture to thetube and in decreasing the mixture shock sensibility.

The optimized formulation for the thermal shock tube is:

-   -   32% to 60% powdered aluminum. Other powdered fuels or reduction        agents able to generate a high temperature spark, such as        magnesium, silicon, boron and zirconium, could also be used.    -   15% to 35% of powdered ferrous-ferric oxide (Fe₃O₄). Other        substances that in oxidation-reduction reactions generate        products with high thermal conduction and convection, such as        ferric-oxide (Fe₂O₃), ferrous oxide (FeO), cobalt oxide, cupric        oxide (CuO), and cuprous oxide (Cu₂O) can also be used.    -   20% to 40% potassium perchlorate (KClO₄). Other substances of        low temperature of Tammann, which are able to lower the energy        of activation of the pyrotechnic reaction and to generate enough        gaseous volume to propagate through kinks, knots, or tube        restrictions, such as potassium chlorate, potassium nitrate,        ammonium perchlorate, sodium perchlorate, sulfur, and antimony        trisulfide, can also be used.    -   0.5% to 3.0% talc. Other substances able to promote adherence        and to reduce shock and friction sensibility, such as graphite,        can also be used.

Referring now to FIG. 1, the process for the production of a thermalshock tube is as follows:

-   -   a) The oxidizers and the adherence promoter and desensitizing        additive are thoroughly mixed, forming mixture I;    -   b) Mixture I is fed into a dosing silo, and the fuels are fed        into another dosing silo;    -   c) The balanced proportions of mixture I and the fuels are        continuously dosed through two parallel dosing thread type        devices or through vibratory dosers or any other conventional        weight or volume microdosing means. The microdosing means        include electric motors with frequency controllers or any other        conventional controller in a control loop with the plastic tube        extruder, so that balanced doses are continuously reaching a        roll homogenizer-mixer with a bottom screen, producing the final        sensitive pyrotechnic mixture in small quantities, the bottom        screen being connected to the extrusion ring of the plastic tube        extruder;    -   d) As the pyrotechnic mixture is being prepared, a melted        polymer is extruded through the extruder ring forming a plastic        tube. While the plastic tube is being formed, the pyrotechnic        mixture is introduced by gravity dosing into the plastic tube.        This yields the desired product, the thermal shock tube.

Additional optional processing steps include tube cooling, stretching ofthe tube to obtain a desired tensile strength, thermal treatment of thetube, and other techniques known in the plastic processing art.

The final product, a thermal shock tube according to the presentinvention, has a conventional plastic tube, such as EVA, POLYETHYLENE,LLDPE or SURLYN, with an outer diameter ranging from 2.0 to 6.0 mm, andan inner diameter ranging from 1.0 to 5.0 mm. The tube includes 5 to 40mg/m of pyrotechnic mixture adhered to its internal walls.

FIG. 2 shows the thermal shock tube spark as it leaves the tip of thetube during propagation. The drawing represents a high velocityphotograph of the tube spark. FIG. 2 shows the high temperature solidand melted products (1), such products including highly thermalconductive and convective melted iron, and the gaseous products (2),which are responsible for the melted jet projection at the tube tip.

FIG. 3 shows, for comparison, the basically gaseous products of aconventional shock tube (prior art) as they leave the tip of the tubeduring propagation. This drawing also represents a high velocityphotograph of the tube flame, and it can be seen that the basicallygaseous products (1) are being dispersed by gas expansion at the tube'send. These comparative drawings (derived from the high speedphotographs) clarify why a conventional shock tube fails to propagatethrough irregularities in the tube and does not have the ability toignite low sensitive delay columns.

The above disclosure is not intended as limiting. Those skilled in theart will recognize that numerous modifications and alterations may bemade while retaining the teachings of the invention. Accordingly, theabove disclosure should be construed as limited only by the restrictionsof the appended claims.

1. A shock tube comprising: an outer tube formed by extrusion, with aninterior region containing a pyrotechnic mixture, said pyrotechnicmixture comprising an oxidizer, a reduction agent, a substance with alow temperature of Tammann, and an adherence promoter.
 2. The shock tubeof claim 1 wherein: said pyrotechnic mixture comprises 32-60% oxidizer,15-35% reduction agent, 20-40% substance with a low temperature ofTammann, and 0.5-3.0% adherence promoter.
 3. The shock tube of claim 1wherein: said oxidizer is from the class of powdered ferrous oxide,ferric oxide, cobalt oxide, cuprous oxide, and cupric oxide.
 4. Theshock tube of claim 1 wherein: said adherence promoter is from the classof talc and graphite.
 5. The shock tube of claim 1 wherein: saiddesensitizing agent is from the class of potassium perchlorate,potassium chlorate, ammonium perchlorate, sodium perchlorate, sulfur,and antimony trisulfide.
 6. The shock tube of claim 2 wherein: saidoxidizer is from the class of powdered ferrous oxide, ferric oxide,cobalt oxide, cuprous oxide, and cupric oxide.
 7. The shock tube ofclaim 2 wherein: said adherence promoter is from the class of talc andgraphite.
 8. The shock tube of claim 2 wherein: said desensitizing agentis from the class of potassium perchlorate, potassium chlorate, ammoniumperchlorate, sodium perchlorate, sulfur, and antimony trisulfide.